FIG. 1 is a cross-sectional view of a wind turbine rotor blade 10. The blade has an outer shell 11, which is fabricated from two half shells: a windward shell 11a and a leeward shell 11b. The shells are moulded from glass-fibre reinforced plastic (GRP). Parts of the outer shell 11 are of sandwich panel construction and comprise a core 12 of lightweight foam (e.g. polyurethane), which is sandwiched between inner 13 and outer 14 GRP layers or ‘skins’.
The blade 10 comprises a first pair of spar caps 15a and 15b and a second pair of spar caps 16a and 16b. The respective pairs of spar caps 15a and 15b, 16a and 16b are arranged between sandwich panel regions 12 of the outer shell 10. One spar cap 15a, 16a of each pair is integrated with the windward shell 11a and the other spar cap 15b, 15b of each pair is integrated with the leeward shell 11b. The spar caps of the respective pairs are mutually opposed and extend longitudinally along the length of the blade 10.
A first longitudinally-extending shear web 17a bridges the first pair of spar caps 15a and 15b and a second longitudinally-extending shear web 17b bridges the second pair of spar caps 16a and 16b. The shear webs 17a and 17b in combination with the spar caps 15a and 15b and 16a and 16b form a pair of I-beam structures, which transfer loads effectively from the rotating blade 10 to the hub of the wind turbine. The spar caps 15a and 15b and 16a and 16b in particular transfer tensile and compressive bending loads, whilst the shear webs 17a and 17b transfer shear stresses in the blade 10.
Each spar cap 15a and 15b and 16 and 16b has a substantially rectangular cross section and is made up of a stack of pre-fabricated reinforcing strips 18. The strips 18 are pultruded strips of carbon-fibre reinforced plastic (CFRP), and are substantially flat and of rectangular cross section. The number of strips 18 in the stack depends upon the thickness of the strips 18 and the required thickness of the shell 11, but typically the strips 18 each have a thickness of a few millimetres and there may typically be between four and twelve strips in the stack. The strips have a high tensile strength, and hence have a high load bearing capacity.
The integration of the spar caps 15a and 15b and 16a and 16b within the structure of the outer shell 11 avoids the need for a separate spar cap such as a reinforcing beam, which is typically bonded to an inner surface of the shell in many conventional wind turbine blades. Other examples of rotor blades having spar caps integral with the shell are described in EP 1 520 983, WO 2006/082479, GB 2497578 and WO 2013/087078.
The wind turbine blade 10 shown in FIG. 1 is made using a resin-infusion (RI) process, whereby the various laminate layers of the shell 11 are laid up in a mould cavity, the cavity is sealed, and a vacuum is applied to the cavity. Resin is then introduced to the cavity, and the vacuum pressure causes the resin to flow over and around the laminate layers and to infuse into the interstitial spaces between the layers. To complete the process, the resin-infused layup is cured to harden the resin and bond the various laminate layers together to form the blade 10.
The pultruded reinforcing strips 18 described above tend to have a relatively smooth and flat outer surface, which is a feature of the pultrusion process. As a result, when the strips are stacked one on top of the other in the mould, there is very little interstitial space between the strips 18. Typically, the space between the strips is between approximately 0.1 and 0.3 mm. This lack of space makes it difficult for resin to infuse between the strips 18, and can result in a poor bond being formed between adjacent strips 18 in the stack. If the strips 18 are not properly bonded together there is a risk of delamination occurring in the blade structure, which may lead to failure of the blade 10 in use. This problem is not limited to pultruded strips 18, but may exist when other types of reinforcing strips having a smooth outer surface are stacked.
One known method for obtaining a surface that is more suitable for bonding is to provide a ‘peel ply’ layer 38 on the upper and lower surfaces of the pultruded reinforcing strip 18 as illustrated in FIG. 2, which can be removed to form a textured surface comprising irregularly arranged peaks and troughs. The textured surface provides space at the interface between the stacked strips 18, allowing resin to infiltrate more easily between the strips 18, for example by capillary action.
Such peel plies 38 are made of a tightly-woven fabric, typically a polyamide, which is coated with a release agent. The tight weave of the fabric and the release agent prevent the peel ply bonding to the resin in the strip, so that the peel ply can be removed easily.
As illustrated in FIG. 3, the peel ply 38 is applied to the strip 18 during the pultrusion process. The peel ply 38 is drawn through a die 40 together with the fibres 42 and the resin that will form the strip 18. The strip 18 is then cured. When the peel ply 38 is removed, it removes a layer of cured resin from the surface of the strip 18, thereby providing the textured surface.
Although the use of peel ply layers 38 aids with resin infusion, there are some problems associated with the use of peel ply layers 38. The peel ply layers 38 must be removed from each strip 18 by hand before the strips 18 are laid in the mould, which is cumbersome and time consuming, and prolongs the manufacturing process
Furthermore, because the surfaces left behind by removal of the peel ply layers 38 are rough and irregular, contact between the strips 18 is similarly irregular. Contact may be peak-to-peak (i.e. between peaks on the textured surfaces of neighbouring strips 18 in the stack), which spaces the strips 18 further apart to provide a relatively large interstitial space in which resin flows relatively easily, or contact may be peak-to-trough (i.e. between a peak on the surface of one strip and a trough on the surface of a neighbouring strip 18 in the stack) which holds the strips 18 closer together to provide a relatively small amount of interstitial space in which resin flow is relatively slow. The amount of interstitial space, and hence the resin flow rate, therefore varies across the strips 18, which can lead to air becoming trapped between the strips 18, which weakens the bond between the strips 18.
It is an object of the invention to mitigate or overcome at least one of the problems of the prior art.